1. Field of the Invention
This invention relates to a signal processing circuit, such as a compressor, limiter or expander, and particularly relates to a signal processing circuit in an audio device which subjects an input signal to variable amplification in accordance with the level of the input signal.
2. Discussion of the Related Art
Electronic audio devices, such as compact-disc (CD) players, digital audio tape (DAT) recorder/reproducers, cassette tape players, and radio tuners, are used to reproduce audio signals from various recording media, or from broadcast radio signals. The quality of the reproduced audio signals heard by a listener, that is the incident auditory characteristics of the audio signals, is generally determined by the content of the reproduced audio signal, and the nature of the audio reproducing environment. The content of the reproduced audio signal can be as varied as the audio components of a single voice as compared with a classical orchestra. Likewise the reproducing environment may vary in size, acoustic quality, and level of ambient background noise.
For example, in a case where a musical audio signal is reproduced in an environment having a high level of background noise, such as a motor car or train, pianissimo portions of the audio signal cannot be heard over the noise. This difficulty may be overcome by increasing the volume, i.e., increasing the gain of the audio signal amplifying circuits, but this method is often not practical because any loud portions of the audio signal are reproduced too loudly. Other environments exist where generally increasing the volume of the audio signal is unacceptable. For example, musical audio signals may be objectionable to persons other than the listener in quiet environments such as public buildings, trains or elevators. In such instances, audio signals must be reproduced at modest or low volumes, and the pianissimo portions of the audio signals may become difficult to hear.
Conventional audio devices have applied a variety of signal processing circuits in various attempts to eliminate the above-described difficulties. These signal processing circuits include a compressor for compressing the dynamic range of an output signal, a limiter for limiting the maximum level of the output signal, and an expander for expanding the dynamic range of the output signal.
FIG. 7 illustrates an example of a compressor found in a conventional audio device. In the compressor 70, amplification gain of a variable gain control section 72 connected to a signal line 71 is varied according to the level of an input signal which is detected with an input signal level detecting section 73. Signal compression is typically performed according to an input-output characteristic, such as the one shown by the solid line 81 in FIG. 8. In other words, the gain of variable gain control section 72 is increased or decreased according to the level of the input signal, such that the dynamic range of the output signal is compressed. As a result of compressor operation, musical audio signals having low volume are increased to be heard over interfering background noises, and musical audio signals having high volume are decreased so as not to be offensive to the ear. Thus, the difficulty accompanying musical audio signals having portions with widely variable volumes is somewhat eliminated.
In the conventional compressor circuit having the input-output characteristic shown in FIG. 8, when the input signal level changes from S.sub.1 to S.sub.2, and back to S.sub.1, the amplification gain is changed from B.sub.1 to B.sub.2, and back to B.sub.1. The switching amplification gain is effected by a time constant which is determined by the circuitry arrangement and circuit elements used in the conventional compressor. The general relationship between switching amplification gain and attack and release time is shown in FIG. 9 .
Attack (or rise) time for increasing amplification gain to a first predetermined value should be controlled as follows: In a case where input signal level is high, and in order to eliminate the problem of output signal saturation or clipping, it is necessary to reduce the attack time to suppress the input signal level as quickly as possible. In a case where input signal level is low, and in order to eliminate the problem of mushy sounding reproduced audio signals, it is necessary to increase the attack time to improve audio crispness resulting from the attack at the rise of the signal.
In addition, release (or fall) time for decreasing amplification gain to a second predetermined value should be controlled as follows: In general and in order to respond to a high signal level as quickly as possible, when the input signal level is high, the release time should be somewhat short, whereas when the input signal level is low, the release time should be relatively longer so that the audio signals are reproduced to sound natural.
Conventional compressor designs have not fully taken into account the above-described requirements. In contrast, amplification gain in conventional compressors is switched with attack and release time constants typically determined by the circuitry arrangement and circuit elements used in the compressor. The above-described problems are apparent not only in conventional compressor designs, but also in other conventional signal processing circuits such as a limiters and expanders.